Dell may have a serious issue on its hands with their E6400 and E6500 laptops.
Hundreds of reports have been surfacing on the official dell support forums stating that they have had "massive performance issues...whenever the devices get slightly warm the BIOS automatically starts dialling down the performance until they basically crawl to a halt, some indicating that even when cool they won't go over 50% of maximum clock".
Computer issues are nothing new and many manufactures have had their fair share of problems in the past, but what makes this one interesting is that Dell will not acknowledge the problem; they have gone as far as trying to cover up the issue by banning members.
The most prominent member of the Dell support forums, Tinkerdude, created a PDF entitled "Performance loss during normal operation in a Dell Latitude E6500 laptop due to processor and bus clock throttling" that explains in detail the problem and potential solutions. The user was banned after this PDF was posted to the forum which leads many to believe that Dell does not want this problem to be known publicly.
Update: It appears that the issue may be due to an underpowered PSU. The laptops ship with a 90 watt PSU but need a 130 watt PSU, likewise, the downgraded performance was only occurring while running on AC power. If you have one of these laptops and are using a 90 watt power supply, you may want to contact Dell.
Update #2: Dell has released several updates that supposedly will remedy the situation mentioned above; although, other reports are starting to surface that the Alienware m15x and Studio XPS 1645 may be suffering from similar issues of throttling. Lionel from Dell support also commented on this report (see below) and states "We're aware of concerns raised in this post and others like it. At this point, our teams are looking into the details. When we have more information to share, we'll update customers via a post on Dell's blog, Direct2Dell."
Thanks for the update tip amrit